Image sensing device and method of

ABSTRACT

A two-dimensional, temporally modulated electromagnetic wavefield, preferably in the ultraviolet, visible or infrared spectral range, can be locally detected and demodulated with one or more sensing elements. Each sensing element consists of a resistive, transparent electrode (E) on top of an insulated layer (O) that is produced over a semiconducting substrate whose surface is electrically kept in depletion. The electrode (E) is connected with two or more contacts (C 1 ; C 2 ) to a number of clock voltages that are operated synchronously with the frequency of the modulated wavefield. In the electrode and in the semiconducting substrate lateral electric fields are created that separate and transport photogenerated charge pairs in the semiconductor to respective diffusions (D 1 ; D 2 ) close to the contacts (C 1 ; C 2 ). By repetitively storing and accumulating photocharges in the diffusions (D 1 ; D 2 ), electrical signals are generated that are subsequently read out for the determination of local phase shift, amplitude and offset of the modulated wavefield.

The invention relates to an image sensor element. The invention furtherrelates to a device for and a method of detection and demodulation of amodulated wavefield. The invention still further relates to a method ofdetermining the three dimensional shape of a reflective object.

The present invention may be applied to all sensing and measuringtechniques that require the sensitive local detection and demodulationof temporally modulated electromagnetic wavefields, preferably in theultraviolet, visible or infrared spectral range. This capability isuseful, in particular, for non-contact distance measurement techniquesbased on optical phase-shifting interferometry or on time-of-flightranging. The present invention may be applied, in particular, to allsensing and measurement techniques that require dense one- ortwo-dimensional arrays of demodulation pixels.

DE 44 40 613 C1 teaches the detection and demodulation of intensitymodulated wavefields with sensing elements that consist of three parts:one photosensitive part, in which incident photons are converted into aproportional number of electronic charge pairs, one or more storageelements, into which the photogenerated charges are stored andaccumulated and an equal number of switches between the photosensitivepart and each storage element. The switches are operated synchronouslywith the modulation frequency. A preferred embodiment relies on chargecoupled device (CCD) techniques, as described by A. J. P. Theuwissen in“Solid-state imaging with charge-coupled devices”, Kluwer, Dord-recht,1995. There the photosensitive site and the switches are realized andoperated as CCD gates that transport the photogenerated chargelaterally. Disadvantages of this approach include the limiteddemodulation speed that is obtained with CCDs, especially if largephotosensitive sites and CCD gates are employed, the necessity forspecial semiconducting processes for the fabrication of the CCDstructures, and the demands on clocking waveforms with specially shapedrising or falling edges in order to obtain a high charge transferefficiency under the CCD gates.

An alternative embodiment of the switches employs field effecttransistors. (FETs), as available in industry standard ComplementaryMetal Oxide Semiconductor (CMOS) processes. This type of switch issimpler to operate, and it is readily fabricated. The disadvantage ofthe FET switch is increased charge and voltage noise behaviour due toincomplete charge transfer, charge injection effects and channel currentnoise caused by gate voltage fluctuations.

DE 198 21 974 A1 claims to overcome the speed limitations of largephotosensitive elements by replacing the single large photogate with acomb-like structure of interdigitated finger electrode photogates. Thephotogenerated charge carriers are therefore more rapidly collected, andthey can also be transferred more quickly on two or more storageelements. This invention relies also on switching elements fortransferring photocharge onto suitable storage elements. Thedisadvantages of these switching elements, realized as CCD gates ofFETs, are the same as described for DE 44 40 613 C1.

EP 00109721 describes an alternative sensing element for the detectionand demodulation of intensity modulated wavefields. It employs twophotosensing parts per sensing element, each with two storage sites andassociated switching element. When used in conjunction with a diffusingoptical component on top of the sensing element for the equaldistribution of the incoming wavefield intensity on the two photosites,this device allows prolonged integration times and relieves the timingrestrictions on the clock waveform. The number of storage sites islimited to four, rendering this device ineffective if more than foursamples per period of the modulated waveform should be taken. Since thisinvention also relies on switches for the transfer of photocharges fromthe photosites to the storage elements the same disadvantages areencountered as described for the above two inventions.

CH 3176/96 teaches the use of a resistive, elongated electrode with astatic voltage difference at the two ends, as a means forphotogenerating and transporting charge carriers with improved speedalong one lateral direction. This is achieved with the static lateralelectric field that is created parallel to the surface at thesemiconductor-insulator interface. This lateral field moves photochargessignificantly faster compared with a conventional CCD structure that hasan electrode of the same size but not resorting to the lateralelectrical field disclosed in this invention. Since photocharge can onlybe moved in one fixed direction, no demodulation action for an incidentmodulated wavefield can be obtained with such a device.

U.S. Pat. No. 5,528,643 describes even faster lateral transport ofphotogenerated charge carriers, by employing a series of CCD gates, eachof which has contacts at both ends at which voltage differences can beapplied. In this way, each CCD electrode exhibits a lateral drift fieldat the semiconductor-insulator interface. The object of the inventiondisclosed in U.S. Pat. No. 5,528,643 is the architecture of atwo-dimensional CCD image sensor with improved photocharge transportspeed in the column and read-out line directions. Since photocharge canonly be moved in one fixed direction, no demodulation action for anincident modulated wavefield can be obtained with such a device.

One object of the invention is to provide a new optoelectronic sensingdevice for the local demodulation of a modulated electromagneticwavefield, preferentially in the ultraviolet, the visible and the nearinfrared portion of the electromagnetic spectrum.

A further object of invention is to provide an architecture forgeometrical arrangement of the sensing device in one or two dimensionsfor the realization of demodulation line and image sensors.

In a first aspect the invention provides an image sensor elementcomprising a semiconductor substrate, a radiation transparent insulatinglayer formed on the semiconductor substrate, an electrode formed as alayer of transparent resistive material on the insulating layer, a firstcontact adjacent to one edge of the resistive layer, a firstdiffusion-region in the semiconductor substrate of opposite conductivityto the semiconductor substrate located adjacent to the first contact andbiassed to a higher potential than that of the first contact, a seconddiffusion region is the semiconductor substrate of opposite conductivityto the semiconductor substrate located adjacent to the second contactand biassed to a higher potential than that of the second contact, meansfor applying an electrical potential between the first and secondcontacts, and means for reading out the charge on the first and/orsecond diffusion regions.

Further preferred, advantageous, and alternative features of the imagesensor element are disclosed and claimed in the dependent claims 2 to 14to which reference should now be made.

Thus an image sensor element is provided that consists of a resistive,transparent electrode on top of an insulated layer that is produced overa semiconducting substrate whose surface is electrically kept indepletion. The electrode is provided with two or more contacts by whichit may be connected to a number of clock voltages that are operatedsynchronously with the frequency of a modulated wavefield. In theelectrode and in the semiconducting substrate lateral electric fieldsare created that separate and transport photogenerated charge pairs inthe semiconductor to respective diffusions close to the contacts. Byrepetitively storing and accumulating photocharges in the diffusions,electrical signals may be generated that are subsequently read out forthe determination of local phase shift, amplitude and offset of themodulated wavefield.

In a second aspect the invention provides a device for the detection anddemodulation of a modulated wavefield comprising an image sensorconsisting of a one or two dimensional array of image sensor elements,each image sensor element being an image sensor element according to theinvention; a signal generator for supplying time dependant voltagepatterns to the contacts on each of the image sensor element electrodesin synchronism with the modulation frequency of the incident wavefieldto transport photocharges laterally to the corresponding diffusions onwhich photocharges are accumulated; and readout means for reading outthe charges on the diffusions for use in calculating the modulationparameters of the incident modulated wavefield.

Further preferred, advantageous, and/or alternative features of thedevice for the detection and demodulation of a modulated wavefield aredisclosed and claimed in dependant claims 16 to 18 to which referenceshould now be made.

The demodulation device according to the present invention mitigatesdisadvantages of the state of the art devices in several respects: themodulation and demodulation frequencies can be increased through theexplicit use of lateral electric fields for the faster transport ofphotogenerated charge carriers to the storage sites. The device consistsof only two elements: the contacted, resistive, transparent electrodeand the charge storage sites, obviating the need for electronicswitches. The device is therefore simpler to operate, since no timingand voltage shaping restrictions must be respected, in contrast to CCDs.For example; in its simplest realization, just one digital clock signalsuffices for proper operation. The device is simple to fabricate byemploying standard CMOS process technology since no overlappingpolysilicon electrodes or buried channels as are required in contrast tocertain CCDs.

In a third aspect the invention provides a method of detecting anddemodulating modulated wavefields comprising the steps of:

-   -   a) illuminating the array of image sensing elements of a device        for the detection and demodulation of a modulated wavefield        according to the invention with the modulated wavefield;    -   b) dividing each period of the modulation frequency into a        number of intervals;    -   c) providing a separate contact and corresponding diffusion        region for each time interval;    -   d) transporting photogenerated charge to the corresponding        diffusion regions during each time interval and storing them        therein;    -   e) reading out the stored charges from the diffusion regions;        and    -   f) calculating demodulation parameters from the charges readout        from the diffusion regions.

In a fourth aspect the invention provides a method of determining thethree dimensional shape of reflective object comprising the steps of:

-   -   a) illuminating the object with a modulated light source;    -   b) imaging light reflected from the object onto an array of        image sensor elements of a device for the detection and        demodulation a modulated wafefield to form a two dimensional        intensity modulated wavefield whose local phase represents local        distance from the object to the detection device;    -   c) dividing each period of the modulation frequency into a        number of time intervals;    -   d) providing a separate contact and corresponding diffusion        region for each time interval;    -   e) transporting photoregenerated charge to the corresponding        diffusion regions during each time interval and storing them        therein;    -   f) reading out the stored charge from the diffusion regions;    -   g) calculating the local phase of the modulated wavefield        incident on the array; and    -   h) using the local phase information to determine the three        dimensional shape of the object.

The above and other features and advantages of the invention will beapparent from the following description, by way of example, ofembodiments of the invention with reference to the accompanyingdrawings, in which:

FIG. 1 shows a first embodiment of an image sensor element according tothe invention,

FIG. 2 shows two embodiments of charges integration circuits for readingcharge from the sensor elements,

FIG. 3 illustrates the operation of the image sensor element of FIG. 1when supplied with a clock signal and when supplied with two anti-phaseclock signals,

FIG. 4 shows a second embodiment of an image sensor element according tothe invention comprising modifications of the element shown in FIG. 1with different electrode contact arrangements,

FIG. 5 shows an embodiment of a four tap demodulation image sensoraccording to the invention, and

FIG. 6 shows the image sensing part of a device according to theinvention for the detection and demodulation of a modulated wavefield.

The present invention makes use of semiconducting material such assilicon for the conversion of incident photons into electron-hole pairs.Without loss of generality, it is assumed in the following that thissemiconducting material is p-doped, and that it is desired to detectelectrons as minority charge carriers in the semiconducting material.All subsequent arguments can be suitably modified to hold true for thedetection of photogenerated holes as minority carriers in n-dopedsemiconducting material.

The semiconducting material is covered with a transparent insulatinglayer, preferentially an oxide, as available in industry standard CMOSprocesses. The thickness of this insulator is preferably between 1 nmand 1 μm. Thinner insulators let a larger part of electric surfacefields into the semiconductor but these thinner oxides are moredifficult to fabricate. On top of the insulator an electrode surface isformed from a transparent, resistive material with an electrical sheetresistivity greater than 10 Ω/square. A preferred material for therealization of this electrode is poly-crystalline silicon. Thegeometrical shape of the electrode is arbitrary, although in practicerectangular shapes are preferred.

The electrode is contacted at its periphery with two or more contactsthat are connected to static and switchable voltage sources. When thesemiconductor material is kept at ground potential and the contactvoltages are positive, the silicon-insulator interface is kept ininversion, so that the photogenerated electrons can be collected andtransported there.

Applying different voltages at the resistive electrode's contacts willlead to a two-dimensional distribution of currents and an associatedtwo-dimensional potential distribution that can be calculated accordingto the laws of electrostatics, as explained in J. D. Jackson, “ClassicalElectrodynamics”, 2nd edition, J. Wiley and Sons, New York, 1975. Thisnon-uniform potential distribution acts across the insulator and createsa corresponding non-uniform potential distribution at thesemiconductor-insulator interface. FIG. 1 illustrates this in a simpleone-dimensional case, leading to a triangular potential distribution atthe interface. Such a non-uniform potential distribution is associatedwith the presence of an electric field parallel to thesemiconductor-insulator interface, given by the negative gradient (orderivative in one dimension) of the potential. Close to each contact atthe electrode's periphery a diffusion region of the opposite conductancetype to the silicon material is created. Since these diffusions have thetask of collecting and accumulating the photocharges, they must bebiased to a higher potential than the corresponding electrode contact.FIG. 1 shows a cross section of the demodulation device with twoelectrode contacts C1 and C2 to the resistive transparent electrode E ontop of the transparent insulator (usually an oxide) O. A voltagedifference between C1 and C2 results in the lateral triangular shape ofthe electronic potential distribution (x) at the semiconductor-insulatorinterface between charge collection diffusions D1 and D2. If the voltageat C1 is higher than at C2, photogenerated electrons are transported bythe triangular surface potential to the diffusion D1. The p-typesemiconductor is held at ground potential with the substrate contact S.The semiconductor near the surface is depleted, down to the edge of thedepletion zone illustrated in FIG. 1 by DZ.

The photocharges on the diffusions can be read out with known electroniccircuits such as the charge integration circuit illustrated in FIG. 2 a.The charge integration circuit shown in FIG. 2 a is based on anoperational amplifier with capacitive feedback C, whose positive inputterminal is held at the reference potential V_(1,2). The photocurrent Ioriginates from a diffusion of the photosensitive device, and it resultsin an output voltage V. The integration process can be reset and startedfrom zero by closing the switch SC. FIG. 2 b illustrates an alternativein the form of an active pixel sensor circuit. The input terminal U_(E)is connected to one of the storage diffusions of the photosensitivedevice. The voltage that is generated by the photocharge on thecapacitance of the storage diffusion acts on the base of the sourcefollower transistor T. The charge integration process can be reset toreference voltage U_(DC) with reset transistor T_(RS). The sourcefollower transistor T, whose drain is connected to the supply voltageU_(DC) and whose source is connected to the load resistor R, producesthe output voltage U_(A).

Incident light is transmitted through the transparent electrode and thetransparent insulator into the semiconducting material whereelectron-hole pairs are created near the semiconductor-insulatorsurface. Electrons diffuse through the semiconducting material untilthey feel the electric field of the depletion region near the surface,forcing them to move to the semiconductor-insulator interface. At thisinterface the strong lateral electric field, generated by the overlyingresistive electrode, sweeps the electrons in the direction where thepotential of the electrode contact is highest. Since the diffusionnearby has even higher potential, the electrons are attracted to thisdiffusion area where they are stored and accumulated. As a consequence,all photoelectrons under the electrode drift rapidly to this diffusionwhere they are all collected and stored.

The incident light is temporally modulated with a given frequency f,exhibiting a period T=1/f. For the operation of the demodulation deviceaccording to this invention, the period T is divided into two or moretime intervals. For each time interval, another voltage configuration atthe electrode contacts is generated with a suitable electronic timingcircuit, employing for example a field programmable gate array (FPGA).Each voltage configuration has the property that another electrodecontact has the highest potential. During this time interval,photogenerated electrons are moved to the corresponding storagediffusion where they are stored and accumulated.

The above described sequence of operations can be repeated for manyperiods, during a long total exposure time, before the photochargesaccumulated in the diffusions are electronically read out. This permitsan increase in the number of detected photoelectrons in the diffusion,and an increase in the corresponding signal-to-noise ratio.

The result of the described operation is two or more electrical signalvalues, one for each storage diffusion, that are available at the end ofeach total exposure time.

These signal values are then used to calculate the modulationparameters, i.e. to carry out the desired demodulation.

As an example, two signal values S1 and S2, sampled from a modulatedwavefield at times that differ by half of the modulation period, allowthe calculation of the phase P of a sinusoidally modulated, offset-freeincident wavefield by the equation P=arc sin((S1−S2)/(S1+S2)).

As a further example, four signal values S1, S2, S3 and S4, sampled froma modulated wavefield at times that differ by a quarter of themodulation period, allow the calculation of the phase P of asinusoidally modulated incident wavefield by the equation P=arc tan((S4−S2)/(S1−S3)).

A plurality of the detection and demodulation devices according to theinvention can be arranged in one or in two dimensions, resulting indemodulation line sensors or demodulation image sensors. Each of thedetection and demodulation devices must be provided with at least thefollowing set of electrical connections:

-   -   Power supply voltage and ground    -   One input voltage line for each of the electrode's contacts that        are switched synchronously with the modulation frequency    -   One reset signals for resetting the electronic charge detection        circuit after the signals have been read out and a new exposure        and charge accumulation period starts    -   One reset reference voltage line, providing the potential value        to which the charge storage and accumulation diffusions are        discharged during the reset operation.    -   One pixel selection line that allows the selection of the pixels        whose signals should be read out and/or reset.    -   One output signal for each charge detection circuit that is        connected with the corresponding charge storage diffusion. The        pixel select line connects the output signals to one or several        busses that are common to several pixels, typically to a        complete column. Alternatively fewer bus lines than diffusion        signals may be provided; in this case a demultiplexing circuit        can distribute these signals on the available busses. This makes        it necessary to provide each pixel with the appropriate lines        for controlling the demultiplexing circuit.

It is possible to displace the position of the transport region, inwhich the photoelectrons are transported laterally to the respectivecharge storage and accumulation diffusions, from thesemiconductor-insulator interface into the bulk of the semiconductor.The method is known from buried channel CCDs, and it is described in A.J. P. Theuwissen in “Solid-state imaging with charge-coupled devices”,Kluwer, Dord-recht, 1995. This is achieved by fabricating an area of theopposite doping type of the semiconductor at the surface and bycompletely depleting this area with a suitable voltage. In this way, thetransported charge carriers are majority charge carriers but since theymove in the bulk of a completely depleted semiconductor, they benefitfrom very efficient transport properties and negligible losses. Typicaldepth values for this buried transportation channel are between 10 and1000 nm.

It is also possible to enhance the sensitivity of the detection anddemodulation device according to this invention for wavefieldsconsisting of photons with energies close to the band gap of thesemiconductor. It is known that photons with such long wavelengths (inthe near infrared for silicon) penetrate deeper into the semiconductor,to a depth where no electric field normally reaches. For this reason,photogenerated charge must rely on a thermal diffusion mechanism toreach the surface, where electric fields are available for fast drifttransports. The thermal diffusion mechanism is slow, since the transporttime depends, on average, on the square of the distance to be travelled.For this reason it is desirable to adapt the demodulation deviceaccording to this invention to make it suitable also for applicationwith long-wavelength photons. This is achieved by fabricating an area ofthe opposite doping type of the semiconductor at the surface and bycompletely depleting this area with a suitable voltage. In this way, thetransported charge carriers are majority charge carriers but since theymove in the bulk of a completely depleted semiconductor, they benefitfrom very efficient transport properties and negligible losses, asdescribed above, following the principles known from buried channelCCDs. The complete circuits for controlling and reading out the pixelsignals are also fabricated in such areas of opposite doping type. Allof these areas are electrically connected to ground potential. Thesemiconductor substrate is biased to a highly negative voltage ofseveral tens of Volts in the case of a p-type substrate. In this way,the depletion region in the semiconductor substrate extends deeply intothe semiconductor bulk, to depths of several tens of micrometers. Inthis mode, called “deep depletion”, vertical electric fields extenddeeply into the semiconductor, leading to fast and efficient drifttransport of photogenerated charges also for longer wavelengths of theincident photons.

If a sinusoidally modulated wavefield is not overlaid by a signal ofconstant value, i.e. if the wavefield is offset-free, then it issufficient for the extraction of the modulation amplitude and the phasedelay to measure two signals per demodulation device. Such ademodulation pixel is preferably realized as a rectangular electrodewith two contacts and two corresponding charge storage and accumulationdiffusions on opposite sides or corners of the electrode. A crosssection of such a two-tap device is shown in FIG. 1.

This device can be operated either with one or with two clock signals.The simpler way of operating this device with one clock signal only isillustrated in FIG. 3 a. One contact, for example C1, is kept at aconstant intermediate voltage level, while the other contact C2 isconnected to a clock signal that switches between a high and a lowvoltage level. During the first half T1 of the clock period thephotoelectrons move to the left charge storage and accumulationdiffusion D1, during the second half T2 of the clock period thephotoelectrons move to the right charge storage and accumulationdiffusion D2.

The device can also be operated with two counter-phase clock signals asillustrated in FIG. 2 b. The contacts C1 and C2 are connected to twoseparate clock signals that switch between a high and a low voltagelevel. To provide for the necessary lateral field in the device, theclock signals must be selected so that one clock signal is at its highvoltage level, while the other is at its low voltage level. The twoclock signals that are in opposite phase generate an electrical fieldthat is twice as large as in case of only one clock signal, so that thecharge carriers are moved with double the speed to the respective chargestorage and accumulation diffusions D1 and D2.

In the general case, a sinusoidally modulated wavefield is characterizedat each position with three values: the modulation amplitude, the phasedelay and the offset value. For this reason, the detection anddemodulation pixel for such a general modulated wavefield requires atleast three contacts on the electrode, three corresponding chargestorage and accumulation diffusions and three clock signals that changethree times per period of the modulation frequency. If four instead ofthree signals are employed, then the demodulation equations areparticularly simple, and for this reason, a four-tap device according tothis invention represents a preferred embodiment. Three examples of suchfour-tap pixels are shown in FIG. 4, illustrating the possibilities forfabricating the contacts either as squares of minimum size or aselongated structures, fabricating the diffusions on the four sides of arectangular electrode, fabricating the diffusions at the corners of arectangular electrode, and fabricating several diffusions on the sameside of an electrode. The electrode shape can be arbitrary but forpractical reasons rectangular shapes are preferred in semiconductortechnology. FIG. 4 shows preferred embodiments of the demodulationdevice with four electrode contacts C1, C2, C3 and C4 on the electrode Eand corresponding storage diffusions D1, D2, D3 and D4 (top view). FIG.4 a shows storage diffusions situated at the four sides of therectangular electrode E, FIG. 4 b shows storage diffusions situated atthe corners of the rectangular electrode E, and FIG. 4 c shows storagediffusions situated two each on two sides of the rectangular electrode.

A preferred implementation of the voltage signals that create thelateral drift fields in a four-tap demodulation pixel is illustrated inthe following table.

V1 V2 V3 V4 T0 H I L I T1 I H I L T2 L I H I T3 I L I H

During the four times T0, T1, T2, and T3, each lasting a quarter of thetotal period T of one charge collection and accumulation sequence, adifferent voltage pattern V1, V2, V3, and V4 is applied to the fourcontacts C1, C2, C3 and C4. In the table H represents a high voltagelevel, L represents a low voltage level and I represents an intermediatevoltage level between H and L. The contact whose corresponding diffusionshould collect the electrons during a certain time receives the highestvoltage. The contact that is opposite the collection contact receivesthe lowest voltage. The other two contacts receive an intermediatevoltage that is typically halfway between the two extreme voltages. Inthis way, maximum lateral electric fields are created beneath theelectrode. The voltage signals that are applied to the contacts have thesame period as the modulation frequency, and each voltage signal is justa phase-delayed copy of a master signal. It is not even necessary thatthese signals are step functions, it is also possible that all contactsignals are sinusoids with a phase delay of a quarter of the periodbetween each contact.

The general detection and demodulation device according to thisinvention consists of one or more electrodes, each with two or morecontacts and the same number of corresponding diffusions. In the case ofone electrode with n contacts and n corresponding diffusions (an n-tapdemodulation pixel), it is possible to detect and demodulate incidentmodulated wavefields whose modulation waveform is described with nparameters. An example of such a demodulation is a waveform that is alinear combination of n/2 sine signals each with its proper amplitudeand n/2 cosine signals each with its proper amplitude. An n-tapdemodulation pixel collects all the signals that are necessary for ademodulation operation that is mathematically carried out by a discreteFourier transform, as explained for example in D. W. Kammler, “A FirstCourse in Fourier Analysis”, Prentice Hall, New Jersey, 2000.

A preferred embodiment of a complete 4-tap demodulation image sensor isillustrated in FIG. 5. The elementary picture element (pixel) is shownin FIG. 5. Each of the sensing element's storage diffusions D1 . . . D4is connected to the gate of a source follower transistor S1 . . . S4,whose drain is kept at the supply voltage VS. The diffusions can bereset to the reference potential VD with the reset transistors R1 . . .R4, employing the reset signal line VR. The source follower transistorsare connected with row select transistors T1 . . . T4 to the bus linesV1 . . . V4 that are common to all pixels in a column. The row selecttransistors pass the signals from the source follower transistors to thebus lines under control of the row select signal line RS. In order toprovide a proper electrical ground potential to each pixel, a groundline G common to all pixels is employed.

FIG. 6 illustrates how these elementary pixels P are arranged intwo-dimensional fashion in the active image sensing part IS of acomplete demodulation image sensor. The row select signal for each rowof pixels is provided by a row select address generator. The resetvoltage VR for all pixels is provided by a reset signal generator. Thefour electrode contacts of each pixel obtain their signals from thevertical contact signal lines C1 . . . C4, which are driven by anelectrode contact voltage pattern generator. All pixels in a row, whoseaddress has been selected by the row select address generator, supplythe output signals of their diffusion source followers to the verticalsignal lines V1 . . . V4. Each vertical signal line V1 . . . V4 isterminated with an active load transistor B1 . . . B4, whose gate iskept at the common bias voltage VB. The voltage signals of the verticalsignal lines V1 . . . V4 are amplified by the column amplifiers A1 . . .A4. These amplifiers feed their signals through a multiplexingtransistor M1 . . . M4 into a common multiplexed readout line MX. Themultiplexing transistors M1 . . . M4 are switched on or off by a columnselect address generator. The signal on the line MX is amplified by theamplifier AA and is delivered to the output line O.

The present invention can be employed, for example, for the opticalmeasurement of the three-dimensional shape of an object according to thetime-of-flight ranging technique, as described in R. Lange and P. Seitz,“Solid-State Time-of-Flight Range Camera”, IEEE J. Quantum Electronics,Vol. 37 (3), 390-397, 1 Mar. 2001, the contents of which is herebyincorporated by reference. An object is illuminated with a modulatedsource of light, and the reflected light is imaged with an opticalimaging lens on a two-dimensional detection and demodulation deviceaccording to this invention. The reflected light forms a two-dimensionalintensity modulated wavefield, whose local phase delay carries theinformation about the local distance of the object to the detection anddemodulation device, since light travels at a finite speed through airof about c=310⁸ m/s. The present invention allows the measurement of allmodulation parameters of the incident modulated wavefield, in particularthe local phase delay t. With this measurement, the local distance L tothe object, and thereby also its three-dimensional shape, is determinedaccording to the equation L=ct/2.

Features of various embodiments of the invention are set forth in thefollowing numbered paragraphs.

-   -   1. Device for the detection and demodulation of a modulated        wavefield with the following properties:        -   An image sensor that consists of a one- or two-dimensional            arrangement of sensing elements        -   Each sensing element consists of one or more photosensitive            parts that convert incoming photons of the wavefield into            charge carriers. Each photosensitive part is provided with            an overlaying resistive electrode layer, provided with one            or more contacts, with which lateral electric fields are            created for the lateral transport of photocharge. A storage            element is placed close to each contact, the storage element            being protected from the incident wavefield, where            photocharge is collected, accumulated and stored.        -   Each storage element is provided with an electronic readout            circuit, through which the stored photocharge signal can be            accessed and read out.        -   Each storage element is provided with a reset switch through            which the voltage at the storage element can be reset to a            reference voltage. In the case of a readout circuit that            measures the photocurrent while keeping the storage element            at a virtual reference voltage, no such reset switch is            required.        -   An electronic generator that supplies time-dependent voltage            patterns to the contacts, in synchronicity with the            modulation frequency of the incident wavefield, so that the            created photocharge is transported laterally to the            corresponding storage elements, where the photocharge is            collected and accumulated for one or several periods of the            modulation frequency. These accumulated photocharge signals            are then read out, and they are used for the calculation of            the modulation parameters of the incident modulated            wavefield.        -   An electronic generator that supplies the signals to the            sensing elements and their readout circuits that are            required for the sequential readout of the photocharge            signals.    -   2. In a device as set forth in paragraph 1, the photosensitive        part may be implemented as a piece of semiconducting material,        covered by a transparent insulating layer, on top of which a        transparent resistive electrode is placed, to which electrical        contacts are fabricated in different places, so that current can        pass from one contact to another. In the semiconducting        material, diffusions are fabricated close to the electrical        contact locations, realized as highly doped areas of the        opposite conduction type than the semiconducting material.    -   3. In a device as set forth in paragraph 1 and/or 2, the        photosensitive part may be implemented with a semiconducting        layer at the surface and of opposite conduction type to the        semiconducting substrate material, this surface layer being        biased with a voltage so that it is fully depleted.    -   4. In a device as set forth in paragraph 3, the photosensitive        part and all electronic circuits may be implemented in surface        semiconducting layers, all of these semiconducting layers being        connected to ground potential, while the semiconductor substrate        is connected to a voltage that forms a so-called deep depletion        layer in the semiconductor substrate. For p-type semiconductor        substrate the substrate voltage must be largely negative, for        n-type semiconductor substrate the substrate voltage must be        largely positive.    -   5. In a device as set forth in any of paragraphs 1 to 4, the        readout electronics may be implemented as a source-follower with        pixel-select transistor, as known from active pixel sensor (APS)        image sensors.    -   6. Alternatively, in a device as set forth in any of paragraphs        1 to 4, the readout electronics may be implemented as a        resettable charge-amplifier with pixel-select transistor, or as        a transconductance amplifier with pixel-select transistor for        measuring the photocurrent at the storage element.    -   7 In a method for the detection and demodulation of modulated        wavefields using devices as set forth in any of paragraphs 1 to        6        -   The wavefield is incident on the detection and demodulation            elements, either directly or through optical elements        -   The wavefield creates photocharges in the photosensitive            parts of the detection and demodulation elements, whose            numbers are dependent on the temporally changing intensity            of the wavefield.        -   Each period of the modulation frequency is separated into            temporal intervals, for each of which a separate contact and            a storage diffusion are available. During each temporal            interval, photogenerated charge is transported to the            corresponding storage element, where the photocharge is            collected, accumulated and stored. Photocharge transport            occurs under the influence of a lateral electrical field            that is provided by the voltages at the contacts and the            thereby produced currents in a resistive electrode layer.            These voltages may be generated by a voltage generator that            functions synchronously with the modulation frequency. The            voltages are generated such that the electrode, into whose            corresponding storage element photocharge should be            transported, is supplied with the most attractive voltage of            all electrodes; this voltage is positive for photogenerated            electrons, and it is negative for photogenerated holes.            Photocharges are transported from the electrodes to the            nearby storage elements by biassing the storage elements            with an even more attractive bias voltage that may be            provided through reset switches that carry out periodic            reset and biassing operations.        -   In a first phase, photocharges are repetitively collected            and stored in the corresponding storage elements for one or            more periods of the modulation frequency of the incident            wavefield.        -   In a second phase, the stored photocharges are read out            sequentially, by employing the electronic readout circuits            with which the storage elements are provided. The read out            stored charges represent the signals with which the            modulation parameters of the incident modulated wavefield            can be calculated by an evaluation unit.    -   8. Using a method as set forth in paragraph 7, the        three-dimensional shape of a reflective object may be        determined. An object is illuminated with a modulated source of        light, and the reflected light is imaged with an optical imaging        lens on a two-dimensional detection and demodulation device as        set forth in paragraphs 1 to 6. The reflected light forms a        two-dimensional intensity modulated wavefield, whose local phase        carries the information about the local distance of the object        to the detection and demodulation device. This method allows the        measurement of all modulation parameters of the incident        modulated wavefield, in particular the local phase. With this        parameter, the local distance to the object and its        three-dimensional shape can be determined.

1. An image sensor element comprising: a semiconductor substrate, aradiation transparent insulating layer formed on the semiconductorsubstrate, an electrode formed as a layer of transparent resistivematerial on the insulating layer, the transparent resistive materialextending across a photosensitive part of the image sensor element inwhich incident light is converted into photogenerated charges, a firstcontact adjacent to a first edge of the resistive layer, a firstdiffusion region in the semiconductor substrate of opposite conductivityto the semiconductor substrate located adjacent to the first contact andbiased to a higher potential than that of the first contact, a secondcontact adjacent to a second edge, opposite the first edge, of theresistive layer, a second diffusion region is in the semiconductorsubstrate of opposite conductivity to the semiconductor substratelocated adjacent to the second contact and biased to a higher potentialthan that of the second contact, means for applying an electricalpotential between the first and second contacts, and means for readingout the charge on the first and/or second diffusion regions; wherein theresistive layer is rectangular; and in which the contacts are arrangedone at each side.
 2. The image sensor element as claimed in claim 1,comprising four contacts each having a diffusion region adjacentthereto.
 3. The image sensor element as claimed in claim 2, in which thecontacts are arranged one at each corner.
 4. The image sensor element asclaimed in claim 2, in which two contacts are arranged on each of twoopposite sides.
 5. The image sensor element as claimed in claim 1, inwhich the resistive layer is square.
 6. The image sensor element asclaimed in claim 1, in which the insulating layer is between 1 nanometer(nm) and 1 micrometer (μm) thick.
 7. The image sensor element as claimedin claim 1, in which the electrode has a sheet resistivity of greaterthan 10 Ohms (Ω)/square.
 8. The image sensor element as claimed in claim1, in which the photosensitive part of the element is implemented in asemiconducting layer at the surface of the substrate, the surfacesemiconducting layer being of opposite conductivity to the substrate,the element further comprising means for biasing the surfacesemiconducting layer so that it is fully depleted.
 9. The image sensorelement as claimed in claim 8, in which the readout means is implementedin the surface semiconductor layer, the surface semiconductor layer isarranged to be connected to ground potential, and the semiconductorsubstrate is arranged to be connected to a potential such as to producea deep depletion layer in the semiconductor substrate.
 10. The imagesensor element as claimed in claim 1, in which the read out means isimplemented as a source follower with a pixel select transistor.
 11. Theimage sensor element as claimed in claim 1, in which the readout meansis implemented as a resettable charge amplifier with a pixel selecttransistor.
 12. The image sensor element as claimed in claim 1, in whichthe readout means is implemented as a transconductance amplifier, formeasuring the photocurrent at the first or second diffusion regions,with a pixel select transistor.
 13. A device for the detection anddemodulation of a modulated wavefield, comprising: an image sensorincluding a one or two dimensional array of image sensor elements formedon a semiconductor substrate, each image sensor element comprising: aradiation transparent insulating layer formed on the semiconductorsubstrate, an electrode formed as a layer of transparent resistivematerial on the insulating layer, the transparent resistive materialextending across a photosensitive part of the image sensor element inwhich incident light is converted into photogenerated charges, a firstcontact adjacent to one a first edge of the resistive layer, a firstdiffusion region in the semiconductor substrate of opposite conductivityto the semiconductor substrate located adjacent to the first contact andbiased to a higher potential than that of the first contact, a secondcontact adjacent to a second edge, opposite the first edge, of theresistive layer, and a second diffusion region is in the semiconductorsubstrate of opposite conductivity to the semiconductor substratelocated adjacent to the second contact and biased to a higher potentialthan that of the second contact; a signal generator for supplying timedependent voltage patterns to the contacts on each of the image sensorelement electrodes in synchronism with the modulation frequency of theincident wavefield to transport photocharges laterally from thephotosensitive part of each of the image sensor elements to thecorresponding diffusions on which photocharges are accumulated; and areadout circuit for reading out the charges on the diffusions for use incalculating the modulation parameters of the incident modulated wavefield; wherein the resistive layer is rectangular; and in which thecontacts are arranged one at each side.
 14. The device as claimed inclaim 13, in which photocharges are accumulated over a plurality ofperiods of the modulation frequency of the incident wavefield.
 15. Thedevice as claimed in claim 13, in which each period of the modulationfrequency is divided into a number of time intervals; wherein a separatecontact and diffusion region is provided in each image sensor elementfor each time interval.
 16. The device as claimed in claim 13,comprising an evaluation unit for calculating the modulation parametersof the incident wavefield from the charges readout from the diffusions.17. The image sensor element as claimed in claim 13, in which theinsulating layer is between 1 nanometer (nm) and 1 micrometer (μ) thick.18. The image sensor element as claimed in claim 13, in which theelectrode has a sheet resistivity of greater than 10 Ohms (Ω)/square.19. A method of detecting and demodulating modulated wavefieldscomprising the steps of: a) illuminating an array of image sensingelements with the modulated wavefield, wherein each of the image sensingelements comprises: a radiation transparent insulating layer, anelectrode formed as a layer of transparent resistive material on theinsulating layer, the transparent resistive material extending across aphotosensitive part of the image sensor element in which incident lightis converted into photogenerated charges, a first contact adjacent toone a first edge of the resistive layer, a first diffusion regionlocated adjacent to the first contact, a second contact adjacent to asecond edge, opposite the first edge, of the resistive layer, and asecond diffusion region located adjacent to the second contact; b)dividing each period of the modulation frequency into a number ofintervals; c) providing a separate contact and corresponding diffusionregion for each time interval; d) transporting photoregenerated chargefrom the photosensitive part to the corresponding diffusion regionsduring each time interval and storing them therein; e) reading out thestored charges from the diffusion regions; and f) calculatingdemodulation parameters from the charges readout from the diffusionregions.
 20. The image sensor element as claim in claim 19, wherein theresistive layer is rectangular.
 21. The image sensor element as claim inclaim 19, in which the contacts are arranged one at each side.
 22. Themethod as claimed in claim 19, in which charges are accumulated in thediffusion regions over more than one period of the modulation frequency.23. The method as claimed in claim 19, in which the wavefield isdirected onto the array by optical elements.
 24. The image sensorelement as claimed in claim 19, in which the insulating layer is between1 nanometer (nm) and 1 micrometer (Ω) thick.
 25. The image sensorelement as claimed in claim 19, in which the electrode has a sheetresistivity of greater than 10 Ohms (Ω)/square.
 26. A method ofdetermining the three dimensional shape of a reflective objectcomprising the steps of: a) illuminating the object with a modulatedlight source; b) imaging light reflected from the object onto an arrayof image sensor elements to form a two dimensional intensity modulatedwavefield whose local phase represents local distance from the object tothe detection device, wherein each of the image sensing elementscomprises: a radiation transparent insulating layer, an electrode formedas a layer of transparent resistive material on the insulating layer,the transparent resistive material extending across a photosensitivepart of the image sensor element in which incident light is convertedinto photogenerated charges, a first contact adjacent to one a firstedge of the resistive layer, a first diffusion region located adjacentto the first contact, a second contact adjacent to a second edge,opposite the first edge, of the resistive layer, and a second diffusionregion located adjacent to the second contact; c) dividing each periodof the modulation frequency into a number of time intervals; d)providing a separate contact and corresponding diffusion region for eachtime interval; e) transporting photoregenerated charges to thecorresponding diffusion regions by applying a potential across the firstcontact and the second contact during each time interval and storingthem therein; f) reading out the stored photogenerated charges from thediffusion regions; g) calculating the local phase of the modulatedwavefield incident on the array; and h) using the local phaseinformation to determine the three dimensional shape of the object. 27.The image sensor element as claimed in claim 26, in which the insulatinglayer is between 1 nanometer (nm) and 1 micrometer (Ω) thick.
 28. Theimage sensor element as claimed in claim 26, in which the electrode hasa sheet resistivity of greater than 10 Ohms (Ω)/square.
 29. An imagesensor element comprising: a semiconductor substrate, a radiationtransparent insulating layer formed on the semiconductor substrate, anelectrode formed as a layer of transparent resistive material on theinsulating layer, the transparent resistive material extending across aphotosensitive part of the image sensor element in which incident lightis converted into photogenerated charges, a first contact adjacent to afirst edge of the resistive layer, a first diffusion region in thesemiconductor substrate of opposite conductivity to the semiconductorsubstrate located adjacent to the first contact, a second contactadjacent to a second edge, opposite the first edge, of the resistivelayer, a second diffusion region in the semiconductor substrate ofopposite conductivity to the semiconductor substrate located adjacent tothe second contact, a voltage generator for applying an electricalpotential between the first and second contacts, and a read-out circuitfor reading out the photogenerated charges on the first and/or seconddiffusion regions; wherein the resistive layer is rectangular; and inwhich the contacts are arranged one at each side.
 30. The image sensorelement as claimed in claim 29, comprising four contacts each having adiffusion region adjacent thereto.
 31. The image sensor element asclaimed in claim 30, in which the contacts are arranged one at eachcorner.
 32. The image sensor element as claimed in claim 29, in whichtwo contacts are arranged on each of two opposite sides.
 33. The imagesensor element as claimed in claim 29, in which the resistive layer issquare.
 34. The image sensor element as claimed in claim 29, in whichthe insulating layer is between 1 nanometer(nm) and 1 micrometer (Ω)thick.
 35. The image sensor element as claimed in claim 29, in which theelectrode has a sheet resistivity of greater than 10 Ohms (Ω)/square.36. The image sensor element as claimed in claim 29, in which thephotosensitive part of the element is implemented in a semiconductinglayer at the surface of the substrate, the surface semiconducting layerbeing of opposite conductivity to the substrate, the element furtherbeing biased to deplete the surface semiconducting layer.